Generally, there have been known so-called rotary atomizing head type coating machines which are constituted of an air motor which is connected to an atomizing head, a speed sensor for detection of rotational speed of the air motor, an air source for supplying driving air to the air motor, an electropneumatic converter for adjusting a supply air pressure from the air source according to an electrical quantity, and a controller for controlling an electrical quantity to be output to the electropneumatic converter on the basis of detected rotational speed and a target rotational speed (e.g., Japanese Patent Laid-Open No. 2002-192022).
In the case of the prior art rotary atomizing head type coating machines of this sort, by way of a feedback control of an air motor, an electrical quantity to be applied to an electropneumatic converter is adjusted by a controller in such a way as to minimize a difference between a detected rotational speed and a target rotational speed of an air motor. Therefore, in the case of conventional rotary atomizing head type coating machines, for example, an air motor is driven at a speed which is within a differential range of ±5% relative to a target rotational speed of approximately 3,000 rpm to 1,000 rpm thereby putting the rotary atomizing head in high speed rotation while supplying a paint to the rotary atomizing head in this condition. As a result, the supplied paint to the rotary atomizing head is atomized by rotary atomization (by centrifugal atomization) to form finely divided paint particles. Atomized paint particles are charged by a rotary atomizing head at an external electrode to urge a flight from the coating machine toward a work piece along an electrostatic field for deposition on the work piece.
In the case of the atomizing head type coating machine by the above-mentioned prior art, an air motor is employed as a drive source for the rotary atomizing head instead of an electric motor. The reasons for this are: (1) High insulating properties of compressed air of the drive source make it easier to insulate the motor as a part to be applied with a high voltage; (2) Relatively simple construction permits reductions in size and cost and inexpensive maintenance and service; and (3) No possibilities of volatile and flammable organic solvent and paint taking fire within the motor.
However, an air motor has a relatively small torque so that the rotational speed of the motor is easily fluctuated by variations in load conditions of the rotary atomizing head (the air motor), for example, when paint supply is turned on and off. On such an occasion, if the rotational speed of the atomizing head is increased, paint is divided into particles of a smaller diameter. On the contrary, if the rotational speed is lowered, paint is divided into particles of a larger diameter. In this connection, it is important to maintain paint particles in a uniform size because the paint particle size has great influences on the quality of finish touches. On the other hand, the rotational speed of the atomizing head varies as the paint supply is switched on and off, making it difficult to atomize paint into an aimed particle size because of impairing the quality of coatings.
Especially, in recent years, it is usually the case for a coating machine to switch paint supply on and off repeatedly several tens times per one car body while coating same according to shapes of its exterior surfaces. Besides, due to a demand from coating industries, there has been a trend toward high paint discharge operations using a paint with a large content of a highly viscous non-volatile component of a high specific gravity. As a consequence, the rotational speed is fluctuated to a greater degree by on-off of paint supply, and deviated from a target rotational speed for a longer time (e.g., for 7 to 10 seconds). In addition, fluctuations in rotational speed take place several tens times per one car body, each time disturbing the paint particle size which has extremely great influences on the quality of coatings.